1. Field of the Invention
The present invention relates to data compressing and expanding apparatus which compress and expand a received signal with high efficiency.
2. Description of the Related Art
Conventionally, a differential pulse code modulation (DPCM) system and an adaptive differential pulse code modulation (ADPCM) system are proposed as a high efficiency coding system which saves the transmission capacity and reduces the capacity of a memory which records signals by compressing and expanding signal data.
With a regular ADPCM system, if the original signal is, for example, PCM data of 16 bits, the differential value between adjacent sampled values is obtained with an accuracy of 16 bits and quantized, for example, with 8 bits.
Thereafter, for example, as shown in FIG. 15, the 8-bit differential value is subjected to adaptive quantization with 4 bits containing, for example, a sign bit, by a conversion table having a quantizing step width corresponding to the differential value. FIG. 15 illustrates application of a table A to a small differential value and a table C to a larger differential value.
The differential value, thus subjected to adaptive quantization, is stored in a memory or sent to the receiving end through a transmission path.
While the above is the basic principles of the ADPCM encoding, decoding is performed in a process substantially reverse to the encoding to thereby reproduce the original signal.
In such a case, quantization of a 16-bit accuracy differential value with 8 bits naturally generates a new quantizing error involved in the 16-bit original data.
In order to reduce this error, if the 16-bit differential value is quantized, for example, with 10 bits which includes two additional bits, the resulting quantizing error is reduced, but the adaptive quantized value resulting from the conversion of the differential value by a table would increase in the number of bits, for example, from 4 bits used so far, to 5 or 6 bits, to thereby increase the transmission capacity and memory capacity, undesirably.